The present invention generally relates to apparatus and methods for obtaining electric drives with minimal speed variation and, more specifically, to apparatus and methods for using induction machines connected to a variable frequency power distribution system to obtain reduced speed variation in electric drives.
Power electronics for aerospace applications plays a significant role in the modern aircraft and spacecraft industry. This is particularly true in the area of more electric architecture (MEA) for aircraft and military ground vehicles.
The commercial aircraft business is moving toward non-bleed-air environmental control systems (ECS's), variable-frequency (VF) power distribution systems, and electrical actuation. Typical examples are the latest designs, such as the Boeing 787 and the Airbus super jumbo A380. The next-generation Boeing airplane (replacement of the 737) and the Airbus airplanes (replacement for the A320 and A350) will most likely use MEA.
Some military aircraft already utilize MEA, including using MEA for primary and secondary flight control. Military ground vehicles have migrated toward hybrid electric technology where the main power generation and propulsion employs electric machinery and associated power electronics. Therefore, substantial demand for high-performance electric drives has arisen.
In this environment, a need exists for electric drives for aircraft and ground military businesses for the following reasons: increased power levels to handle increased loads; reduced weights to accommodate large content increase per platform; reduced volume to accommodate electronics and electric machines in limited space; increased reliability; and reduced cost.
The power distribution systems for commercial and some military airplanes have been using constant-frequency 400 Hertz (Hz), 115-V AC power. The constant frequency created an opportunity to use an induction machine directly connected to the bus to obtain constant-speed electric drives. Hence, simple electric drives without solid-state power electronics are obtained, resulting in low weight and cost and high reliability. However, the most recent developments for MEA have changed the distribution systems to variable frequency. The speed of the induction machine connected to such a bus will follow the frequency variation, which in some cases is in a ratio of 1 to 2.5. To mitigate this problem to an acceptable speed variation, a power electronics converter with dual-conversion AC to DC and DC to AC is typically implemented. However the cost, weight, volume, and reliability of power electronics are still not affordable for application in variable frequency situations. In addition, additional forced cooling is often required to prevent the power electronics from overheating.
As can be seen, there is a need in the aerospace industry for a reliable low-cost electric drive that may be powered via a variable-frequency power distribution bus without the use of dual power conversion.